1. Field of Invention
The present invention relates to a lead frame, a resin-encapsulated semiconductor device and a fabrication process for the device and particularly, to a lead frame that is applied to a semiconductor element, the element and its periphery being resin-encapsulated while the rear surface of a die pad is exposed to the external environment, an resin-encapsulated semiconductor device and a fabrication process for the device.
2. Description of Related Art
There have been known a conventional resin-encapsulated semiconductor device (a resin-encapsulater semiconductor device will hereinafter be referred to as a semiconductor device) in which the rear surface of a die pad is exposed to the external environment, for example, as shown in FIG. 4, which is a perspective view of appearance of the semiconductor device as viewed from the front side thereof, in FIG. 5, which is a perspective view of appearance of the semiconductor device as viewed from the rear side thereof and in FIG. 6, which is a sectional view taken on line II--II of FIG. 4.
That is, the semiconductor device 10 comprises a lead frame 11 including: a die pad 12; support bars 13 that are protruded outwardly from sides of the die pad 12; inner leads 14; and outer leads 15, wherein the constituents are separated from frame selvages each having a band like shape.
Not only is a semiconductor element (a semiconductor chip) 16 made to firmly adhere to a surface of the die pad 12 of the lead frame 11 with silver paste applied therebetween, but electrodes (not shown) of the semiconductor element 16 are respectively connected to the inner leads 14 using gold wires 17. Further, the semiconductor element 16 is packaged with a construction in which an encapsulating section 18 made of a resin material that encapsulates the semiconductor device 16 and its periphery is formed while the outer leads 15 and the rear surface of the die pad 12 are both exposed to the external environment.
In fabrication of the semiconductor device 10, though not shown in the figures, the lead frame is provided which comprises the die pad 12, bar supports 13 and inner leads 14 all of which are formed in an opening inside the frame selvages having a band-like shape, and the outer leads 15 that are connected to the frame selvages. At first, the semiconductor element 16 is made to firmly adhere to a surface of the die pad of the lead frame with silver paste.
Following the adhesion of the element, the electrodes of the semiconductor element 16 and the inner leads 14 are connected by the gold wires 17 and thereafter, the semiconductor element 16 and the periphery thereof are encapsulated with a resin material while the outer leads 15 and the rear surface of the die pad 12 are exposed to the external environment, so that the device is packaged. Further, the outer leads 15 are press cut using a dedicated lead shaping metal mold so as to be separated from the frame selvage, thereby completing the semiconductor device 10.
This fabricated semiconductor device 10 is mounted on a printed wiring board by soldering the rear surface of the die pad 12 and part of the outer leads 15, which are exposed to the external environment, to the printed wiring board to obtain a printed circuit board. In the semiconductor device 10 mounted on the printed wiring board, since the rear surface of the die pad 12 is directly soldered on the printed wiring board, heat that the semiconductor element 16 generates can be dissipated through the die pad 12 and the printed circuit board.
In a conventional semiconductor device 10 in which the rear surface of the die pad 12 is exposed to the external environment, however, the rear surface of the die pad 12 and the rear surface of the encapsulating section 18 made of a resin material are formed to be coplanar with each other. Hence, in a resin-encapsulating step of semiconductor device 10 fabrication, a flash 19 of the resin material can have a chance to generate on the rear surface of the die pad as shown in FIG. 5 by an encapsulation pressure.
That is, in the resin encapsulating step, the lead frame on which the semiconductor element 16 is mounted and which is still in one piece with the frame selvage is disposed in a cavity formed by a pair of an upper metal mold half and a lower metal mold half in assembly, for example, so that the rear surface of the die pad 12 is oppositely in direct plane contact with the top surface of the lower meal mold half with no resin material covering the rear surface of the die pad 12. Further, outside the periphery of the die pad 12 in such an arrangement, a space is formed in which the resin material can be filled up to the top surface of the lower metal mold half at a position where the rear surface of the die pad is located.
On the other hand, when a lead frame is clamped by the upper and lower metal mold halves, a strain that works to deform the die pad 12 is imposed on the die pad 12 through the support bars 13 of the lead frame, thereby entailing deformation of die pad 12. Therefore, the flash 19 of the resin material occurs on the rear surface of the die pad 12 by an encapsulating pressure when the resin material is injected into the cavity. It should be noted that while description is herein made of the example in which the rear surface of the die pad 12 is put into plane contact with the top surface of the lower metal mold half, another encapsulating metal mold may also be adopted in the encapsulation in which, contrary to this example, the rear surface of the die pad 12 is put into plane contact with the top surface of the upper encapsulating metal mold half wherein the lead frame is placed in the cavity of the mold interchanging positions of the upper and lower sides. In the latter case as well, similar to the former case, a flash 19 of the resin material generates on the rear surface of the die pad 12 by an encapsulating pressure.
If such a flash 19 of a resin material onto the rear surface of the die pad 12 occurs, an exposure area of the rear surface of the die pad 12 is reduced and thereby a level of exposure is lowered. Therefore, when the semiconductor device 10 is actually mounted on a printed wiring board, the rear surface of the die pad 12 is raised while being separated from the mounting surface of the printed wiring board in the portion corresponding to the flash 19 and an inconvenience occurs because of reduction in mechanical strength of a soldered joint, decrease in heat dissipation and others. As a result, problems arise because of reduction in reliability of an electronic component fabricated with the device and a poor production yield thereof.
Further, in a case where the semiconductor device 10 is actually mounted on a printed wiring board, there are two ways: the die pad 12 and the support bars 13 for the die pad 12 are both soldered to the printed wiring board and one of the die pad 12 and the support bars 13 is soldered thereto. However, since the rear surfaces of the die pad 12 and the support bars 13 are coplanar with each other, if the latter case is selected and, for example, only the support bars 13 are soldered, the solder on the support bars 13 flows out to the die pad 12 side with the result that an inconvenience also arises since a sufficient mechanical strength of the soldered joint cannot be attained.